Carbonaceous materials, such as wood, contain mineral components, known as ash, that do not combust when burned in air, regardless of combustion temperature. Alkaline minerals, formed from alkali or alkali earth metals, form one part of the ash. Due to the elevated temperatures achieved during combustion, normally exceeding 850° C., a large proportion of the alkaline minerals are converted from their carbonate form to their oxide form through the loss of carbon dioxide. Wood ash containing primarily the oxide form of alkaline minerals is normally extremely alkaline, with pH values of 12.0 or more, and heavily laden in soluble salts, as evidenced by electrical conductivity values of 20 deciSiemens per meter (dS/m) or more.
Wood ash is applied on land to increase soil pH, buffer soil against decreases in pH due to acid addition, add calcium and magnesium for improved plant growth and soil structure, and increase the supply of micronutrients. Most plants grow optimally in soil having a pH range of from 6.0 to 8.0. The limitations of using wood ash residue, either directly as a soil amendment or as a major component of other fertilizers, are its excessively high pH and high content of soluble salts. For example, wood ash has pH values as high as 13.0, which can cause i) deficiencies in micronutrients such as iron, copper and zinc; ii) ‘toxic shock’ to young seedling plants; and iii) localized mineralogical changes in the soil environment where it is placed. Soluble salt contents of wood ash, measured as electrical conductivity (EC), can reach 40 dS/m. Depending on the amount of wood ash applied per year, these elevated levels of pH and soluble salts can be detrimental to soil quality, crop production, and water quality.
In an article published in Resources, Conservation and Recycling 38 (2003) 301-3116 entitled “Drying of granulated wood ash by flue gas from saw dust and natural gas combustion”, S. L. Homberg, T. Claesson, M. Abul-Milh, and B. M. Steenari; the authors investigated how drying by flue gas affected the chemical composition and properties of wood ash. The conclusion of the Homberg et al is that drying by flue gas was an environmentally acceptable way to dry granules in terms of effects on hardening and the chemical composition of the granules to make them less reactive.